Descriptive statistics with descrip

Throughout our examples, we’ll use the fev dataset. This dataset is included in the rigr package; see its documentation by running ?fev.

## Preparing our R session
library(rigr)

## rigr version 1.0.4: Regression, Inference, and General Data Analysis Tools in R

data(fev)

First, we can obtain default descriptive statistics for the dataset simply by running descrip().

descrip(fev)

##           N     Msng  Mean      Std Dev    Min       25%       Mdn     
## seqnbr:     654     0   327.5     188.9     1.000     164.2     327.5  
## subjid:     654     0   37170     23691     201.0     15811     36071  
##    age:     654     0   9.931     2.954     3.000     8.000     10.00  
##    fev:     654     0   2.637     0.8671    0.7910    1.981     2.547  
## height:     654     0   61.14     5.704     46.00     57.00     61.50  
##    sex:     654     0   1.514     0.5002    1.000     1.000     2.000  
##  smoke:     654     0   1.099     0.2994    1.000     1.000     1.000  
##            75%       Max     
## seqnbr:     490.8     654.0  
## subjid:     53638     90001  
##    age:     12.00     19.00  
##    fev:     3.118     5.793  
## height:     65.50     74.00  
##    sex:     2.000     2.000  
##  smoke:     1.000     2.000

Since we input a dataframe, we can see that all variables have the same number of elements given in the N column. None of our variables have any missing values, as seen in the Msng column.

Rather than specifying the whole dataframe, if we are interested in only the variables fev and height, we can input only those two vectors into the descrip() function, as below.

descrip(fev$fev, fev$height)

##               N     Msng  Mean      Std Dev    Min       25%       Mdn     
##    fev$fev:     654     0   2.637     0.8671    0.7910    1.981     2.547  
## fev$height:     654     0   61.14     5.704     46.00     57.00     61.50  
##                75%       Max     
##    fev$fev:     3.118     5.793  
## fev$height:     65.50     74.00

Descriptive statistics for strata

Suppose we wish to obtain descriptive statistics of the fev and height variables, stratified by smoking status. To do this, we can use the strata parameter in descrip:

descrip(fev$fev, fev$height, strata = fev$smoke)

##                         N     Msng  Mean      Std Dev    Min       25%     
##    fev$fev:  All          654     0   2.637     0.8671    0.7910    1.981  
##    fev$fev:    Str  no    589     0   2.566     0.8505    0.7910    1.920  
##    fev$fev:    Str  yes    65     0   3.277     0.7500    1.694     2.795  
## fev$height:  All          654     0   61.14     5.704     46.00     57.00  
## fev$height:    Str  no    589     0   60.61     5.672     46.00     57.00  
## fev$height:    Str  yes    65     0   65.95     3.193     58.00     63.50  
##                          Mdn       75%       Max     
##    fev$fev:  All          2.547     3.118     5.793  
##    fev$fev:    Str  no    2.465     3.048     5.793  
##    fev$fev:    Str  yes   3.169     3.751     4.872  
## fev$height:  All          61.50     65.50     74.00  
## fev$height:    Str  no    61.00     64.50     74.00  
## fev$height:    Str  yes   66.00     68.00     72.00

In the output, we can see that overall descriptive statistics, as well as descriptive statistics for each stratum (smoke = 1, smoke = 2) are returned in the table.

Descriptive statistics for subsets

Now suppose we only want descriptive statistics for height and FEV for individuals over the age of 10. We first create an indicator variable for age > 10 outside of the descrip() function, and then give this variable to the subset parameter.

greater_10 <- ifelse(fev$age > 10, 1, 0)
descrip(fev$fev, fev$height, subset = greater_10)

##               N     Msng  Mean      Std Dev    Min       25%       Mdn     
##    fev$fev:     264     0   1.708     0.0000    1.708     1.708     1.708  
## fev$height:     264     0   57.00     0.0000    57.00     57.00     57.00  
##                75%       Max     
##    fev$fev:     1.708     1.708  
## fev$height:     57.00     57.00

Above/Below

Suppose we want to know the proportion of individuals with FEV greater than 2, stratified by smoking status. We can use the strata argument as before, in addition to the above parameter to obtain this set of descriptive statistics:

descrip(fev$fev, strata = fev$smoke, above = 2)

##                      N     Msng  Mean      Std Dev    Min       25%     
## fev$fev:  All          654     0   2.637     0.8671    0.7910    1.981  
## fev$fev:    Str  no    589     0   2.566     0.8505    0.7910    1.920  
## fev$fev:    Str  yes    65     0   3.277     0.7500    1.694     2.795  
##                       Mdn       75%       Max      Pr>2     
## fev$fev:  All          2.547     3.118     5.793     0.7446 
## fev$fev:    Str  no    2.465     3.048     5.793     0.7199 
## fev$fev:    Str  yes   3.169     3.751     4.872     0.9692

From the output, we can see that 96.92% of the individuals in this dataset who smoke (smoking status 1) had an FEV greater than 2 L/sec, and 71.99% of the individuals in this dataset who were nonsmokers had an FEV greater than 2 L/sec.